Kerberos working group J.Brezak
Internet Draft Microsoft
Document: draft-brezak-spnego-http-00.txt
Category: Informational
September 2001
HTTP Authentication: SPNEGO Access Authentication
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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1. Abstract
This document describes how MicrosoftÆs Internet Explorer 5.0 and
Internet Information Services 5.0 use Kerberos for security
enhancements of web transactions. The HTTP auth-scheme of
'negotiate' is defined here; when the negotiation results in the
selection of Kerberos, the security services of authentication and
optionally impersonation are performed.
2. Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
The key words "MUST", "MUSTNOT", "REQUIRED", "SHALL", "SHALLNOT",
"SHOULD", "SHOULDNOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [3].
3. Access Authentication
3.1 Reliance on the HTTP/1.1 Specification
This specification is a companion to the HTTP/1.1 specification [4]
and builds on the authentication mechanisms defined in [5]. It uses
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the augmented BNF section 2.1 of that document, and relies on both
the non-terminals defined in that document and other aspects of the
HTTP/1.1 specification.
4. HTTP Negotiate Authentication Scheme
Use of Kerberos is wrapped in an HTTP auth-scheme of "Negotiate".
The auth-params exchanged use data formats defined for use with the
GSS-API [6]. In particular, they follow the formats set for the
SPNEGO [7] and Kerberos [8] "mechanisms" for GSSAPI. The
"Negotiate" auth-scheme calls for the use of SPNEGO GSSAPI tokens
which the specific mechanism type specifies.
4.1 The WWW-Authenticate Response Header
If the server receives a request for an access-protected object, and
an acceptable Authorization header is not sent, the server responds
with a "401 Unauthorized" status code, and a WWW-Authenticate header
as per the framework described in [4]. The negotiate scheme will
operate as follows:
challenge = "Negotiate" auth-data
auth-data = 1#( [gssapi-data] )
The meanings of the values of the directives used above are as
follows:
gssapi-data
If the gss_accept_security_context return a token for the
client, this directive contains is the base64 encoding of an
InitialContextToken as defined in [6].
A status code 200 response can also carry a WWW-Authenticate
response header containing the final leg of a authentication. Before
using the contents of the response, the gssapi-data should be
processed by gss_init_security_context to determine the state of the
security context. If this function indicates success, the response
can be used by the application. Otherwise an appropriate action
based on the authentication status should be.
For example the authentication could have failed on the final leg if
mutual authentication was requested and the server was not able to
prove its identity. In this case, the returned results are suspect.
It is not always possible to mutually authenticate the server before
the HTTP operation. POST methods are in this category.
When the Kerberos Version 5 GSSAPI mechanism [RFC-1964] is being
used, the HTTP server will be using a principal name of the form of
"http/<hostname>".
4.2 The Authorization Request Header
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The client is expected to retry the request, passing an
Authorization header line, which is defined according to the
framework described in [4] utilized as follow:
credentials = "Negotiate" auth-data2
auth-data2 = 1#( gssapi-data )
gssapi-data
This directive contains is the base64 encoding of an
InitialContextToken as defined in [6].
If a directive or its value is improper, or required directives are
missing, the propose response is 400 Bad Request. If a 401
Unauthorized status code is returned, the contents of the WWW-
Authenticate response header is used to continue the authentication
as long as the opaque value is the same.
5. Negotiate Operation Example
The user is logged onto realm A.COM as user@A.COM. The web server is
in realm B using the principal http/server@B.COM. Realm B.COM trusts
Realm A.COM
The client requests an access-protected document from server via a
GET method request. The URI of the document is
"http://www.nowhere.org/dir/index.html".
The first time the client requests the document, no Authorization
header is sent, so the server responds with:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Negotiate
The client will obtain the user credentials using the SPNEGO GSSAPI
mechanism type to identify generate a GSSAPI message to be sent to
the server with a new request, including the following Authorization
header:
Authorization: Negotiate
2a87421000492ade0234568ac0289eca874209af8bc028
The server will decode the gssapi-data and pass this to the SPNEGO
GSSAPI mechanism in the gss_accept_security_context function. The
return value from the gss_accept_security_context function can
indicate the security context is complete and supply final
authentication data to be returned to the client. If the server has
more gssapi data to send to the client to complete the context it is
to be carried in WWW-Authenticate header with the final response.
The response will be sent to the client, including the following
header:
HTTP/1.1 200 Success
WWW-Authenticate: Negotiate ade0234568ac874209af8bc0280289eca
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The client will decode the gssapi-data and supply it to
gss_init_security_context using the context for this server. If the
status is successful from the final gss_init_security_context, the
response can be used by the application.
7. Security Considerations
The SPNEGO HTTP authentication facility is only used to provide
authentication of a user to server. It provides no facilities for
protecting the HTTP headers or data including the Authorization and
WWW-Authenticate headers that are used to implement this mechanism.
This mechanism is not used for HTTP authentication to HTTP proxies.
If an HTTP proxy is used between the client and server, it must take
care to not share authenticated connections between different
authenticated clients to the same server. If this is not honored,
then the server can easily lose track of security context
associations. A proxy that correctly honors client to server
authentication integrity will supply the "Proxy-support: Session-
Based-Authentication" HTTP header to the client in HTTP responses
from the proxy. The client MUSTNOT utilize the SPNEGO HTTP
authentication mechanism through a proxy unless the proxy supplies
this header with the 401 Unauthorized response from the server.
8. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
3 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
4 Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
5 Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach,
P., Luotonen, A., Stewart, L., "HTTP Authentication: Basic and
Digest Access Authentication", RFC 2617, June 1999.
6 Linn, J., "Generic Security Service Application Program Interface,
Version 2", RFC 2078, January 1997.
7 Baize, E., Pinkas, D., "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.
8 Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964,
June 1996.
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10. Author's Addresses
John Brezak
Microsoft
One Microsoft Way
Redmond, Washington
Email: jbrezak@microsoft.com
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